Germ layer specification in a diploblast

As one of the earliest cell fate decisions in the development of most animals 3 major germ layers (ectoderm, endoderm and mesoderm) have evolved, which play crucial functions during the embryonic development. Ectoderm will form the outer cell layer of the embryo, while endoderm and mesoderm will be internalized to generate inside cell layers. For the most part, the three germ layers give rise to distinct tissues and differentiated cell types . In this context, the most fundamental tasks are to understand (A) how the transition of cell states and cell types during germ layer formation is regulated and (B) what are the consequences of changes in the regulation of cell states on animal body plans and their evolution. However, cnidarians (sea anemones, corals, jellyfish), which arose early in animal evolution, are diploblastic, i.e. they consist only of two germ layers, commonly termed ectoderm and endoderm. Hence, they seem to lack the mesoderm, despite sharing many genes that have a crucial role in mesoderm formation in animals with three germ layers. Recent work has, however, suggested that the diploblastic cnidarians might have already a topological segregation of endodermal and mesodermal tissue identities. This project seeks to reveal how these tissue identities are separated from ectoderm in cnidarians in comparison to vertebrates and insects, with the final goal to get insights into the evolution of the three germ layers in vertebrates. To this end, we will compare cnidarians and vertebrates by studying key regulatory proteins that drive the activation of endodermal and mesodermal developmental programs. These studies will shed light into key changes in the regulatory network of the developmental genes during animal evolution allowing us to draw conclusions on how this major transition from rather simple cnidarians to more complex animals occurred.